1. Field of the Invention
The present invention relates to a polarization converting device, a polarized light illuminating system using the device and an image display unit using the device.
2. Related Background Art
FIG. 1 is a constitutional view showing a conventional example of a projection type display unit.
This projection type display unit comprises a light source 1 consisting of a tungsten halogen lamp or metal halide lamp, a reflection mirror 2 for reflecting a part of light emitted from the light source 1, a heat ray cut filter 3 for absorbing or reflecting the heat ray of light incident thereupon directly or via the reflection mirror 2 from the light source 1, a condenser lens 4 for transforming the light after removing the heat ray to parallel light, a polarizing plate 5 for transforming the parallel light to linear polarized light, a liquid crystal light bulb 7 for modulating the linear polarized light in accordance with an image signal, a polarizing plate 8 for transimtting only component lying in a direction of transmission axis for the modulated linear polarized light, and a projection lens system 10 for projecting on larger scale the transmitted linear polarized light on to a screen, not shown.
FIG. 2 is a constitutional view showing another conventional example of a projection type display unit.
This projection type display unit has two polarizing beam splitters 6, 9 disposed before and after a liquid crystal light bulb 7, respectively, instead of two polarizing plates 5, 8 of the projection type display unit as shown in FIG. 1.
The projection type display unit as shown in FIGS. 1 and 2 has such a disadvantage that a linear polarized light component emitted from the light source 1 and transmitted through the polarizing plate 5 or the polarizing beam splitter 6 is only used for lighting the liquid crystal light bulb 7, and a linear polarized light component orthogonal to the above linear polarized light component is lost, so that the utilization efficiency of light is reduced to less than 50%.
A projection type display unit which improves the above disadvantage is that as described in Japanese Laid-Open Application No. 61-90584, as shown in FIG. 3.
In this projection type display unit, parallel light emerging through the condenser lens 4 enters a polarizing beam splitter 11, wherein its P polarized light component Lp is transmitted directly through an action surface 11a (a deposition film formed on a slant plane having two rectangular prisms bonded with each other) of the polarizing beam splitter 11, while its S polarized light component Ls is reflected therefrom at right angle and enters a total reflection prism 12. The S polarized light component Ls is reflected again at right angle by the total reflection prism 12 to emerge therefrom in the same direction as that of the P polarized light component Lp transmitted through the polarizing beam splitter 11. It is noted that the S polarized light component Ls is a polarized light component parallel to the action surface 11a of polarizing beam splitter 11, and the P polarized light component Lp is that orthogonal to the S polarized light component.
On the emergent side of the total reflection prism 12, a .lambda./2 optical phase plate 13 is disposed, in which the S polarized light component Ls emerging from the total reflection prism 12 has its polarized direction rotated 90.degree. by the .lambda./2 optical phase plate 13 and is converted to a P polarized light component Lp*. On the light emergent side of the polarizing beam splitter 11 and the .lambda./2 optical phase plate 13 are disposed wedge lenses 14, 15 for changing the optical path, whereby each optical path for the P polarized light component Lp transmitted through the polarizing beam splitter 11 and the P polarized light component Lp* converted by the .lambda./2 optical phase plate 13 is deflected and built up in such a manner as to intersect at a point P.sub.O on an incident side of the liquid crystal light bulb 7.
Accordingly, this projection type display units has a utilization efficiency of light twice as great as that as shown in FIGS. 1 and 2, because the liquid crystal light bulb 7 is illuminated with both the S polarized light component Ls and the P polarized light component Lp which are separated by the polarizing beam splitter 11.
As to the above projection type display unit as disclosed in Japanese Laid-Open Patent Application No. 61-90584, there is a problem that the light source 1 is usually made of a lamp, which is not a complete point or line light source, so that the light emerging from the condenser lens 4 is not completely parallel, and the P polarized light component Lp and the converted P polarized light component Lp* are not made parallel completely.
That problem will be described below with reference to FIG. 4.
The light emitted from the light source 1 having a finite diameter .phi. is converged by the condenser lens 4 disposed a distance 1 apart therefrom, but the emergent ray therefrom is not rendered a parallel light completely, producing unparallel light dispersed over a range of an angle 2.omega. (.omega.=tan.sup.-1 ((.phi./2)/1)). A light ray .alpha. of Unparallel light is not influenced by the polarizing beam splitter 11, but passes through an interface between the polarizing beam splitter 11 and the prism 12 to enter the .lambda./2 optical phase plate 13 and emerges therefrom as the ray containing both the S polarized light component and the P polarized light component. Also, a light ray .beta. is made the S polarized light component Ls at the polarizing beam splitter 11, in which after reflecting from the total reflection prism 12, it passes through the interface between the polarizing beam splitter 11 and the prism 12, and is reflected again by the polarizing beam splitter 11 so as to exit from the .lambda./2 optical phase plate 13 as the P polarized light component Lp* at a quite different position as indicated by a light ray .beta..sub.1, or be absorbed into an interface of the .lambda./2 optical phase plate 13 or transmitted directly as indicated by a light ray .beta..sub.2 in FIG. 4, thereby being lost.